Standard locking devices for service station gas tanks are mounted to standpipes which typically extend into fillboxes positioned slightly beneath ground level of the service station. Such devices have filler adapters which are threaded to the pipe leading to the gas tank and a filler cap which is secured to the filler adapter by a standard lock. A typical example is shown in U.S. Pat. No. 1,461,400 to Ocheltree, Jr. Unfortunately, these devices are unsatisfactory, since the padlock on the filler cap can be destroyed easily by cutting with a saw or a bolt cutter or breaking by means of a sledge. Other locking devices are fuel tanks, such as shown in U.S. Pat. No. 1,747,205 to White, U.S. Pat. No. 1,908,593 to Fortune and U.S. Pat. No. 1,936,207 to Penn, are constructed so that with adequate purchase and force, they can be forcefully removed from any tank even if it means twisting the end of the standpipe off. Therefore there has been a need to provide a protective device for gasoline standpipes which frustrates quick and easy removal thereof without a proper key and in some embodiments indicates that an attempt at removal has failed.